This invention relates to a process for recovering waste heat from formaldehyde product gas obtained by catalytic reaction of methanol with air.
Reaction of methanol with air in the presence of a silver catalyst to produce formaldehyde according to the so called methanol excess process consists of an endothermic reaction based on dehydrogenation of methanol and an exothermic reaction based on reaction of the resulting hydrogen with the oxygen in the feed air to produce water, and is, on the whole, an exothermic reaction.
Usually, the catalytic reaction of methanol with air in a commercial scale apparatus is carried out by mixing methanol vapor, preheated air and additional steam in a predetermined proportion, introducing the mixture to a reactor and conducting the reaction in the presence of a silver catalyst at a temperature of 550.degree. to 700.degree. C. The formaldehyde contained in the reaction product gas will be decomposed at an elevated temperature in that case, and thus the product gas is immediately quenched to 120.degree. to 200.degree. C. in a waste heat boiler through heat exchange with pressurized water, and then is introduced into an absorption column.
Since the formaldehyde-forming reaction is a reaction to by-produce water, a two-stage absorption column comprising a packed section as a lower stage and a bubble-capped section as an upper stage is generally used as the absorption column from the viewpoints of water balance and absorption efficiency. The aqueous formaldehyde solution withdrawn from the lower stage of the absorption column is cooled by cooling water, etc. in a heat exchanger, and returned to the absorption column. That is, the sensible heat and latent heat of the product gas are not usually recovered and are discharged in the form of waste warm water.
To utilize the heat possessed by the formaldehyde product gas, processes for recovering it as a heat source for concentrating formaldehyde or for removing unreacted methanol from the product gas have been proposed [Japanese Patent Publications Nos. 34-6718 and 48-20527, and Japanese Patent Application Kokai (Laid-open) No. 49-101312]. However, any of these processes is not satisfactory in the effective utilization of the heat possessed by the formaldehyde product gas. To make more effective utilization, processes for recovering it as a heat source for vaporizing methanol have been also proposed (Japanese Patent Publications Nos. 5511653 and 55-30783).
Recent needs for the product formaldehyde have a tendency toward a higher formaldehyde concentration and a lower methanol concentration. For example, the conventional product has 37% by weight of formaldehyde and 2 to 7% by weight of unreacted methanol, whereas the recent needs for the product formaldehyde are 40 to 55% by weight of formaldehyde and 1 to 5% by weight of unreacted methanol. To meet such needs, it is necessary to select a higher air-to-methanol ratio for a feed gas mixture. To use such a higher air-to-methanol ratio, it is necessary to narrow the explosion limit of an air-methanol gas mixture, and one of the measures is addition of an inert gas such as steam, etc. to the air-methanol gas mixture. However, the amount of such additional inert gas, such as steam, must be increased with enhanced reactivity.
An increasing amount of the additional steam will impair the unit consumption of steam for the production of formaldehyde, and also the direct addition of steam to the reaction system will be a possible cause for fouling the catalyst because of deposition of involatile matters entrained by steam from steam steel pipings. Thus, the amount of additional steam is desirably as small as possible.